Field evaluation of a new biphasic rain garden for stormwater flow management and pollutant removal

Field-scale rain gardens were constructed using a novel biphasic concept, involving water movement from a saturated to an unsaturated zone in sequence, for increasing retention time of runoff and improving bioremediation. Hydraulic performance and removal efficiencies of the biphasic rain gardens were evaluated in natural and simulated runoff events. Influent and effluent of two replicate biphasic rain gardens from natural runoff events were monitored during a 2-yr study. Three agricultural runoff events with high concentrations of nutrients (i.e. nitrate and phosphate) and the herbicide, atrazine (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine), were undertaken during the summer of 2008. Five urban runoff events with spiked concentrations of nutrients (i.e. nitrate and phosphate) and herbicides, glyphosate (N-(phosphonomethyl)glycine), dicamba (3,6-dichloro-2-methoxybenzoic acid), and 2,4-d (dichlorophenoxyacetic acid) were applied to the rain gardens during the summer of 2009. Both peak flow and runoff volume were reduced by holding runoff in the rain gardens (mainly in the water saturated zone) until the next runoff event. The created biphasic rain gardens in our study were highly effective in removing nitrate (∼91%), phosphate (∼99%), atrazine (∼90%), dicamba (∼92%), glyphosate (∼99%), and 2,4-d (∼90%) under high levels of pollution loading with simulated runoff events. Increased retention time of runoff pollutants and water-saturated conditions, as determined by design configuration and rainfall size, intensity, and interval, were found to significantly affect overall nutrient and herbicide removal in the biphasic rain gardens.